1. Field of the Invention
The present invention relates to a communication apparatus for optical communication.
2. Description of the Prior Art
FIG. 1 shows the outer appearance of a transmission/reception module as a signal processing means of a conventional optical communication apparatus. Two transmission/reception modules 11 and 13 are optically coupled through an optical fiber cable 20 consisting of two optical fibers 15 and 17. The optical fiber is made of quartz, plastic or the like. The modules 11 and 13 are mounted on and electrically connected to optical communication equipment (not shown) through connectors 23 and 25, respectively, which are arranged on the outer casings of modules 11 and 13. Mutual conversion of electric signals and light signals between the electric circuitry of the transmission/reception modules 11 and 13 and the optical fiber cable 20 is performed by signal converters 27 and 29, respectively. In a transmission/reception module of the type described above, light signals which are obtained by conversion from the electric signals by one module, or light signals to be converted into the electric signals, which are received from the other module are transmitted through the optical fiber bundles 15 and 17. Conversion between light signals and electric signals is not always necessary. For example, the optical shielding means is generally incorporated in order to prevent adverse effects of ambient light. For this reason, transmitting state of the optical fiber cannot easily be monitored. An abnormal transmitting state may be left unnoticed. This also applies to the case wherein light/electric conversion is not involved.
FIG. 2 shows the transmission/reception module 11 shown in FIG. 1 in greater detail. A light signal 31 transmitted from the other module 13 through the optical fiber bundle 15 is received by a light-receiving element 33 and is converted into an electric signal thereby. The electric signal from the light-receiving element 33 is amplified by an amplifier 35 and is supplied to an output pin 41 of the connector 23. A power supply voltage is supplied to the positive voltage terminal of the amplifier 35 through a positive voltage (+Vc) pin 43.
In the transmission system of the transmission/reception module 11, an electric signal is applied across an input pin 45 and a ground pin (GND) 47 of the connector 23. The electric signal is converted into a light signal 53 by a light-emitting diode (LED) 51. The light signal 53 is transmitted to the other module 13 through the optical fiber bundle 17. A resistor 55 interposed between the input pin 45 of the connector 23 and the light-emitting element 51 serves to limit the current flowing in the light-emitting element 51. If an LED for emitting visible light is used as the light-emitting element 51, a current of 10 to 20 mA may flow. This current flows to the ground pin 47 of the connector 23 in the direction indicated by arrow a. To the ground pin 47 are commonly connected the ground pin of the amplifier 35 as well as one terminal of each of the light-receiving element 33 and the light-emitting element 51 which are optically shielded to avoid adverse effects from an external light source. Therefore, the currents of the transmission system and the reception system flow in this commonly connected line.
The circuit components as described above are mounted on a printed circuit board 61. The conductor pattern formed on the printed circuit board 61 to which a current iR of the transmission system flows has a certain resistance. When this resistance is represented by an equivalent resistance Ro, a voltage drop of Eo (=Ro.multidot.iR) is produced across the pattern. Thus, the voltage drop Eo is superposed on the electric signal from the light-receiving element 33 of the reception system to provide an input signal to the amplifier 35. In this manner, the transmission signal adversely affects the reception signal to cause the problem of crosstalk.